An electronic oscillator provides an output signal which varies between two states in a periodic manner (e.g., an oscillation). The two states typically comprise two voltage levels, but may comprise two current levels. Some electric oscillators are constructed to provide two or more outputs. Each output varies between the two states in the same periodic manner, but there is usually a phase difference in their variations. For example, one output may be 90-degrees farther ahead in the periodic variation than another output. Such oscillators are often called multiphased oscillators, polyphase oscillators, or oscillators with multiple-phase outputs.
Many electronic oscillators also include circuitry that allows the frequency of the oscillation to be varied. These are typically called voltage-controlled oscillators (VCOs), in that an input voltage is used to set the value of the oscillation frequency. It is noted that the terminology “voltage-controlled oscillator” is also applied to those electronic oscillators in which an input current, rather than an input voltage, is used to set the oscillation frequency.